1. Field of the Invention
This invention relates to a system for detecting an alignment mark formed on a substrate or the like and effecting the position detection of the substrate, and in particular to a position detecting system suitable for an exposure apparatus for manufacturing semiconductor elements.
2. Description of the Prior Art
The tendency of large scale integrated circuit (LSI) patterns toward minuteness is progressing every year, and reduction projection type exposure apparatuses have become popular as circuit pattern printing apparatuses which satisfy the requirement for the tendency toward minuteness and which are high in productivity. In these apparatuses heretofore used, a reticle pattern several times (e.g., five times) as large as the pattern to be printed on a silicon wafer is reduction-projected by a projection lens and what is printed in a cycle of exposure is an area on the wafer smaller than a square of diagonal length 21 mm. Accordingly, to print the pattern on the whole surface of a wafer having a diameter of the order of 125 mm, the so-called step-and-repeat system whereby a wafer is placed on a stage and moved by a predetermined distance and then exposure is repeated is adopted.
In the manufacture of LSI, several or more layers of patterns are successively formed on a wafer, but unless the superposition error (positional deviation) of the pattern between different layers is of a predetermined value or less, the conductive or insulative state between the layers will not become what is intended and the function of LSI cannot be performed. For example, for a circuit of a minimum line width of 1 .mu.m, only a positional deviation of the order of 0.2 .mu.m at greatest is allowed.
In the reduction projection exposure system, as a method of superposing patterns one upon the other, namely, a method of superposing the projected image of the pattern on a reticle and the already formed pattern on a wafer one upon the other, there are two methods, i.e., the off-axis type method and the through-the-lens (hereinafter referred to as TTL) type method. In any of these two methods, the alignment mark on the wafer is detected and the alignment of the wafer relative to the apparatus is effected, but it is difficult to obtain high superposition accuracy in every layer. This has been caused by the fact that due to the size of the particles of the crystal on the wafer becoming large, the surface of the wafer becomes rough or the reflection factor thereof is irregular and therefore the detected position of the mark of the wafer deviates from the actual position of the mark. Particularly, where the surface of the wafer is covered with a thin film of aluminum, this phenomenon is substantial, and this has led to a disadvantage that the superposition accuracy is substantially reduced.
Also, the ordinary alignment by the TTL system is effected by detecting through a projection lens the patterns already formed on the wafer, namely, the two marks attendant to a chip. The two marks, for example, in accordance with the two-dimensional alignment direction of the wafer, are formed by a linear pattern elongately extending in x direction and a linear pattern elongately extending in y direction orthogonal to the x direction, and they are provided on different portions around the chip. Such two marks are detected by discrete alignment optical systems through the projection lens. The actual wafer alignment procedure has usually comprised substantially positioning a chip to be aligned in the projection area (image field) of the projection lens, thereafter effecting the y direction positioning of a stage so that the y direction position of the mark extending in x direction is aligned by one alignment optical system, and then effecting the x direction positioning of the stage so that the x direction position of the mark extending in y direction is aligned by the other alignment optical system. Accordingly, the x direction and y direction position detections, i.e., two position detecting operations, become necessary, and this has led to a disadvantage that the speed of alignment is limited.